depressed? this might be why. look at the top illustration. it shows how serotonin (small green circles) travel from neuron to neuron. in most brains, enough serotonin makes it across the synaptic cleft to allow a steady and healthy flow. when too little serotonin makes the jump, one can take an serotonin reuptake inhibitor [ssri], which essentiallly keeps the serotonin out in the gap longer (inhibits its reuptake in the originating neuron) so that it has more opportunity to join up with serotonin receptors on the other side of the cleft. so jump, little serotonins, JUMP! make us happy!
the illustration above is the molecular structure of serotonin. just in case you were wondering what this wonderful wee neurotransmitter looked like.